84 



JAMIN ON METALLIC REFLEXION. 



then calculate the equation 



, cos(l+r) 



tan A' = — )- {. 



cos (1— r) 



The expression (90° — 2 A') represents the difference of phase, 

 or (8). This formula applies exactly to silver and steel : it also 

 represents, with a very satisfactory nearness, the following expe- 

 riments performed on two plates of zinc, on which had been im- 

 pressed a different polish in the two series of trials to which they 

 were subjected, which has changed all the results numerically 

 without altering their lavr. 



First Series. — Zinc, ij = 77- 



